Electrical stimulation of the vestibular nerve produces changes in sympa- thetic outflow and blood pressure (Uchino et al., 1970; Ishikawa et al., 1979). In addition, vestibular lesions affect orthostatic reflexes (cardiovascular responses to a change in body position): such lesions greatly impair blood pressure compensation to 60 deg tilt in cats (Doba and Reis, 1974; Huang et al., 1977). The occurrence of vestibule-sympathetic reflexes which counteract orthostatic hypotension is not unexpected; vestibular receptors inform the nervous system about changes in head position, and thus changes in posture. Since the physiological response to orthostatic hypotension is an increase in sympathetic outflow (Gauer and Thron, 1965), it seems practical for inputs which signal a change in posture to influence the autonomic nervous system. Preliminary studies revealed that neurons in at least two brainstem regions known to contain a high density of cardioregulatory neurons, the ventrolateral medulla and the paramedian reticular nucleus, receive vestibular inputs. In addition, inputs from the carotid sinus and labyrinth were shown to converge on single units in these regions. The proposed studies will utilize both electrical and natural vestibular stimulation and will analyze vestibular inputs to brainstem cardioregulatory neurons as well as vestibular-elicited reflexes recorded from sympathetic nerves. These experiments will show which descending pathways convey vestibular signals to sympathetic preganglionic neurons and characterize the inputs to these pathways. This information will reveal which peripheral endorgans (semicircular canals or otolith organs) are important in producing ventibulo-sympathetic reflexes. In addition, the possibility that somatic inputs from the hindlimb converge on neurons that are part of the vestibulo-sympathetic pathway will be considered.